Mixers are contained in many radio transmitters, receivers, and other communication devices. Such mixers operate by combining an RF signal with LO, a periodic signal, producing a mixed output. This mixed output contains signals whose frequencies consist of either the sum or difference between RF and LO frequencies. By filtering the mixed output, an IF signal can be created whose frequency is either higher or lower than the original RF frequency.
As a result of the desire to decrease the size of communication devices, mixer size must also be reduced. Double balanced diode mixers, widely used in the industry, are large. By utilizing FETs instead of diodes, and removing the multiple baluns, size can be reduced without sacrificing performance.
Passive floating FET mixers are known in the art. A floating FET configuration uses an electrically symmetrical FET, whose drain and source terminals float with respect to ground and are coupled to the output port via a balun, as described in Vasile, C. F., Floating GASFET Circuits Offering Unique Signal Processing from DC-EHF, 1985 GOMAC Proceedings, Pp.305-309, 1985, incorporated herein by reference in it's entirety. FIG. 1, taken from Vasile's article, shows a single floating FET 101 configuration. An alternative floating FET configuration which replaces the single transistor with two transistors 201 and 202 in series is shown in FIG. 2, also taken from Vasile's article. U.S. Pat. No. 4,705,967, issued for an invention of Vasile and incorporated herein by reference in it's entirety, discloses a floating multifunction FET circuit that may be used as a mixer, as shown in FIG. 3. The IF is extracted from the same side of the balun 328 as the RF signal. Therefore, the balun 328 in this design must be large to account for both the higher frequency RF signal and the lower frequency IF signal. Additionally, the RF and IF signals are not isolated, resulting in possible distortion of these signals.
Mourant, in U.S. Pat. No. 5,697,092 and in “A Low cost Mixer for Wireless Applications,” 1995 IEEE Microwave Systems Conference pp. 21-24, also discloses similar subject matter pertaining to passive floating FET mixers, each of which is hereby incorporated herein by reference in its entirety. In U.S. Pat. No. 5,697,092, a single, floating FET mixer suitable for compact construction is disclosed, as shown in FIG. 4, taken from Mourant. The balanced IF created in the FET mixer is extracted at the RF balun 403 in an unbalanced manner, being diplexed from the balun center tap. A series inductor 414 and capacitor 415 resonates at the RF frequency and shorts out center tap 412 to ground 406. At low frequencies, the capacitor 415 is an open, allowing the IF 416 to be extracted. The balun 403 is fabricated with primary and secondary windings consisting of planar metal spirals that are interleaved with one another, as described by Mourant, col. 4, lines 11-18. One problem with this design is that adding a centertap to a balun adds steps to the manufacturing process. Additionally, the centertap position must be located, requiring calculation and testing. Another problem is that the IF output leads must cross over portions of the interleaved windings, making it susceptible to parasitic capacitance.